U.S. patent number 4,996,986 [Application Number 07/506,856] was granted by the patent office on 1991-03-05 for implantable medical device for stimulating a physiological function of a living being with adjustable stimulation intensity and method for adjusting the stimulation intensity.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Niels Thomassen.
United States Patent |
4,996,986 |
Thomassen |
March 5, 1991 |
Implantable medical device for stimulating a physiological function
of a living being with adjustable stimulation intensity and method
for adjusting the stimulation intensity
Abstract
An implantable device for stimulating a physiological function
of a living being with a stimulation intensity
calculated/determined in view of the physical activity of the
living being, wherein the spontaneous intensity of the
physiological function is measured during various phases of
spontaneous activity of the physiological function and the measured
spontaneous intensity is compared to the calculated stimulation
intensity, duration between the measured spontaneous intensity and
calculated stimulation intensity causing corrections in an
algorithm used to calculate the stimulation intensity.
Inventors: |
Thomassen; Niels (Espergaerde,
DK) |
Assignee: |
Siemens Aktiengesellschaft
(Berlin and Munich, DE)
|
Family
ID: |
8201212 |
Appl.
No.: |
07/506,856 |
Filed: |
April 9, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Apr 12, 1989 [EP] |
|
|
89106520.3 |
|
Current U.S.
Class: |
607/19 |
Current CPC
Class: |
A61N
1/36542 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61N 001/365 () |
Field of
Search: |
;128/419PG |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Hill, Van Santen, Steadman &
Simpson
Claims
I claim:
1. A medical device implantable into the body of a living being,
comprising:
(a) means for stimulating a physiological function of the living
being with adjustable stimulation intensity;
(b) detector means for detecting phases of spontaneous activity of
the physiological function;
(c) means connected to said detector means for measuring the
spontaneous intensity of said physiological function during the
phases of spontaneous activity;
(d) sensor means forming a signal corresponding to the physical
activity of the living being;
(e) converting means connected to said sensor means and containing
a variable allocation arrangement for converting said signal into a
stimulation intensity correlating to the physical activity;
(f) adjustment means connected to said means for stimulating and
said means for converting for providing said means for stimulating
with said converted stimulation intensity; and
(g) means connected to said means for measuring, said adjustment
means and said converting means for comparing said spontaneous
intensity with said converted stimulation intensity during said
phases of spontaneous activity, and, in case of a deviation, for
adjusting said allocation arrangement to a conversion of said
signal corresponding to the physical activity into a value of said
stimulation intensity, which, at least essentially, corresponds to
the measured spontaneous intensity.
2. The device of claim 1, wherein said allocation arrangement
comprises a function memory in which the converted signals are
stored in the form of a function table that contains defined values
of the stimulation intensity for defined values of the signal of
the sensor means, whereby the allocation arrangement is adjusted by
replacing the existing defined value corresponding to the signal of
the sensor means with the measured value of the spontaneous
intensity.
3. A method for setting the stimulation intensity of a device for
stimulating a physical function of a living being with a
stimulation intensity adapted to the physical activity of the
living being, comprising the steps of:
(a) measuring a spontaneous intensity of the physiological function
during phases of spontaneous activity of the physiological
function;
(b) sensing the physical activity and converting the sensed
physical activity into a stimulation intensity corresponding to the
sensed physical activity;
(c) comparing the spontaneous intensity measured during a phase of
spontaneous activity to the converted stimulation intensity
corresponding to the sensed physical activity during the phase of
spontaneous activity; and
(d) altering the conversion of the sensed physical activity so that
a converted stimulation intensity, converted by the altered
conversion, at least essentially corresponds to the measured
spontaneous intensity.
4. The method of claim 3, wherein the conversion is altered only if
the measured spontaneous intensity falls between defined limits.
Description
BACKGROUND OF THE INVENTION
The invention is directed to medical devices implantable into the
body of a living being having means for stimulating a physiological
function of the being with adjustable stimulation intensity,
comprising a sensor for forming a signal corresponding to the
physical activity of the living being, means for implementing an
algorithm for calculating a stimulation intensity adapted to the
physical activity of the living being, and adjustment means that
sets the stimulation intensity in view of the algorithm. The
invention is also directed to a method for adjusting the
stimulation intensity of the implantable device.
As used herein, the term "stimulation intensity" is understood to
be comprehensive and to include duration, frequency, repetition
rate, amplitude, etc. with which, e.g., means for stimulation are
activated. Thus, the term "stimulation intensity" means any
combination of the above-listed parameters.
Implantable medical devices of the type described above allow a
living being, in whom they are implanted, to lead a normal life
insofar as possible by providing necessary stimulation of a
malfunctioning physiological function. The stimulation is provided
with an intensity that depends upon the physical activity of the
living being, the stimulation intensity corresponding as much as
possible to that intensity that would be present if the living
being were not dependent upon the artificial stimulation of the
physiological function by the implantable medical device.
In U.S. Pat. No. 4,428,378, the teachings of which are fully
incorporated herein by reference, there is disclosed an implantable
medical device of the type discussed above. The disclosed device is
an implantable heart pacemaker. The device includes a
piezo-electric pressure sensor integrated therein that registers
mechanical vibrations in the body of the being arising from
movement of the muscles and the like during physical activities of
the being, these mechanical vibrations propagating as pressure
waves, and that converts the vibrations into a correlated physical
activity electrical signal. The stimulation intensity, i.e., the
stimulation frequency with which the heart pacemaker stimulates the
heart given the absence of natural heartbeats, is calculated with
reference to the correlated physical activity. The calculation is
made according to a predetermined algorithm. The stimulation
intensity is then set via adjustment means.
The algorithm used is based on the characteristics of an average
patient, on the average coupling relationships of the
piezo-electric sensor to the body of the patient, and on the
average manufacturing tolerances with respect to the heart
pacemaker, particularly with respect to those components that are
used in the formation of the electrical signal correlated to the
physical activity. This means that the stimulation frequency that
is set and calculated according to the algorithm only rarely
coincides with the heartbeat frequency with which the heart of the
patient would be at spontaneously given a physical activity. In the
majority of cases, the stimulation frequency that is set for a
defined physical activity can deviate greatly from the spontaneous
heartbeat frequency that the patient would otherwise have
experienced given the physical activity. Thus, it can occur that
the calculated stimulation frequency can correspond well to the
patient's requirements for a time following implementation of the
heart pacemaker, but otherwise deviates more and more from the
spontaneous heartbeat frequency with which the heart of the patient
would beat due to, for example, tissue growing over the
piezo-electric sensor.
SUMMARY OF THE INVENTION
The present invention provides an implantable device for
artificially stimulating a physiological function of a living being
wherein the stimulation intensity is made to correspond closely, if
not exactly, to the natural stimulation intensity that would
otherwise be present given normal physiological function. To that
end, the invention provides an implantable device for artificial
stimulation of a physiological function of a living being, wherein
the artificial stimulation intensity is determined algorithmically
in view of sensed activity of the being and wherein natural
stimulation intensity is measured with respect to the activity to
alter that algorithm used to determine the artificial stimulation
intensity so that the artificial stimulation intensity is made to
correspond closely, if not exactly, to the natural stimulation
intensity that would otherwise be present during such activity
given normal physiological function.
In an embodiment, the invention provides an implantable device for
artificially stimulating a physiological function of a living being
wherein the artificial stimulation intensity is determined
algorithmically in view of sensed activity, including detector
means for detecting phases of spontaneous activity of the
physiological function; means for measuring the spontaneous
intensity of the physiological function; means for comparing the
spontaneous intensity measured during a phase of spontaneous
activity of the physiological function to the artificial
stimulation intensity determined during the phase of spontaneous
activity; and means for altering the algorithm used to determine
the artificial stimulation intensity to eliminate deviation between
the artificial stimulation intensity and the measured spontaneous
intensity.
The invention takes into consideration that the spontaneous natural
stimulation intensity of the physiological function to be
stimulated established for a defined physical activity of the
physiological function is the stimulation intensity that is best
suited to the requirements of the living being. When the
physiological function must be artificially stimulated, the
artificial stimulation intensity should correspond to the
spontaneous natural stimulation intensity present for the same
physically activity during phases of spontaneous activity of the
physiological function.
In an embodiment, the invention provides that the algorithm used to
calculate the artificial stimulation intensity is corrected, i.e.,
altered, so that the calculated artificial stimulation intensity
corresponds to the spontaneous intensity of the physiological
function in view of the physical activity. Thus, the physiological
function is artificially stimulated with a stimulation intensity
that largely, if not exactly, corresponds to that spontaneous
natural stimulation intensity of the physiological function
otherwise present given normal physiological function.
An advantage of the invention therefore is that deviations between
natural and artificial stimulation intensities are eliminated, at
least for the most part. Further, if and when the characteristics
of the device change, (for example, due to age) the algorithm is
altered to compensate for such changes. At the very most,
undesirable deviations occur during relatively short time spans
required for correcting, i.e., altering, the algorithm.
In an embodiment, means for implementing the algorithm comprises a
function memory. It is therefore easy to implement and alter the
algorithm since exchange of data stored in the function memory is
all that is required. Moreover, the data stored in the function
memory is extremely easily accessible. Thus, in an embodiment, the
physical activity signal is digitized and the resulting digital
data is used to address the function memory.
In an embodiment, the invention provides that a check is performed
before a correction of the algorithm takes place to determine if
the measured spontaneous intensity falls within defined limits.
Performance of this check prevents physiological meaningless
correction of the algorithm. For example, the limits can involve
maximum or minimum stimulation intensity levels.
BRIEF DESCRIPTION OF THE FIGURE
The sole figure is a block diagram of a heart pacemaker.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
In the figure there is illustrated a heart pacemaker constructed
for implantation in the body of a living being, e.g., a human
being. The heart pacemaker is designed to work in the VVI mode and,
accordingly, communicates with a heart 2 of the living being via an
electrode 1 introduced into a ventricle of the heart 2.
The electrode 1 supplies a signal corresponding to electrical
activity of the heart 2 to the pacemaker. Within the pacemaker, the
signal supplied by the electrode 1 is received by a detector 3. The
detector 3 detects the occurrence of spontaneous heartbeats. When
an event having a minimum amplitude corresponding to a natural
heartbeat and/or when an event having a specific steepness that is
typical of a natural heartbeat occurs in the signal corresponding
to the electrical activity of the heart 2, the detector means 3
outputs a signal indicating the occurrence of a natural heartbeat
to a control logic circuit 4.
The illustrated heart pacemaker also comprises a stimulation pulse
generator 5 that is in communication with the control logic circuit
4 and can be activated by the control logic circuit 4 to output an
electrical stimulation pulse for stimulating a heartbeat. The
output of the stimulation pulse generator 5 is connected to the
electrode 1. Thus, the electrode not only supplies the signal
corresponding to the electrical activity of the heart 2 to the
heart pacemaker but also conducts the stimulation pulses from the
heart pacemaker to the heart 2.
The control logic circuit 4 always causes the stimulation pulse
generator 5 to output a stimulation pulse when no natural heartbeat
is detected by the detector means 3, after the expiration of a
defined time interval (referred to as the base interval) following
a natural heartbeat detected by the detector means 3 or a
stimulation pulse output by the stimulation pulse generator 5. The
heart pacemaker thus prevents the heartbeat frequency from dropping
below a frequency corresponding to the base interval, since, as
required, it stimulates heart activity with a stimulation frequency
that corresponds to the base interval.
The duration of the base interval is calculated by the control
logic circuit 4. To this end, the control logic circuit 4 counts
off a defined plurality of clock pulses that are supplied to it by
a clock generator 6, for example, a crystal oscillator.
In the case of the described heart pacemaker, the chronological
duration of the base interval does not have a fixed value. On the
contrary, the chronological duration of the base interval varies in
view of the physical activity of the living being such that the
stimulation frequency with which the heart pacemaker stimulates
falls between an upper limit value, for example 150 pulses per
minute, and a lower limit value, for example, 60 pulses per minute.
Increasing physical activity of the living being will cause the
stimulation frequency to increase from the lower limit to the upper
limit.
Data corresponding to the stimulation limits can be stored in the
control logic 4 as values representing stimulation pulses or
heartbeats per minute. The heart pacemaker thus can simulate the
dependency of the spontaneous heartbeat frequency on the physical
activity by setting a stimulation frequency matched to the physical
activity.
In order to be able to calculate the pacemaker stimulation
frequency adapted to the physical activity or, respectively, the
chronological duration of the base interval corresponding thereto,
a piezo-electric pressure sensor 7 is provided that is connected to
the wall of a housing 8 that surrounds the electronics of the heart
pacemaker in hermetically tight fashion. During physical activities
of the living being, mechanical vibrations in the body of the
living being that arise due to the movement of the muscles and the
like, propagate as pressure waves in the body of the living being
and are registered by the pressure sensor 7 and converted into
electrical signals. These signals, whose amplitudes increase with
increasing physical activity, are supplied to a signal editing
circuit 9 that filters and amplifies these signals.
An output signal of the signal editing circuit 9 is supplied to an
analog-to-digital converter 10. The digital output signals thereof
are in turn supplied to address inputs of a write/read memory,
preferably a random access memory (RAM) 11 that normally operates
in read mode. An input W/R of the RAM 11 serving the purpose of
switching the RAM 11 from the write mode to read mode and vice
versa is supplied with a corresponding signal by the control logic
circuit 4.
Data input/outputs of the RAM 11 are connected to a data processing
bus 12 via which, among other things, they are connected to the
control logic 4.
The RAM 11 works as a function memory. To this end, data
corresponding to the stimulation frequency, such as value
representing the number of stimulation pulses or heartbeats per
minute as in the case of the exemplary embodiment, and data
corresponding to the duration of the base interval, are stored in
the RAM 11 for defined values of the digital output data of the
analog-to-digital converter 10 that respectively correspond to a
defined physical activity of the patient. An algorithm for
calculating a stimulation frequency adapted to the physical
activity of the patient respectively existing is thus, so to speak,
stored in the RAM 11 in the form of a function table.
The data stored in the RAM 11 that correspond to the respectively
existing output data of the analog-to-digital converter 10 are
supplied to the control logic circuit 4 that in turn calculates
counts of plurality of clock pulses of the clock generator 6 in
view of these data. The counts correspond to durations of the base
interval adapted to the respective physical activities of the
patient. It should be clear that the stimulation frequency, or,
respectively, the chronological duration of the base interval that
is established, changes according to the data stored in the RAM 11
matched to the physical activity of the life form.
In order to provide that the stimulation frequency set for a
defined physical activity of the patient corresponds as exactly as
possible to that heartbeat frequency with which the heart of the
patient would spontaneously beat given the same physical activity,
it is provided in the illustrated heart pacemaker that the
spontaneous heartbeat frequency is measured during phases of
spontaneous heart activity wherein the heart follows a sinue
rhythm. The spontaneous heartbeat frequency measured during such a
phase is compared to the pacemaker stimulation frequency for that
phase. In the case of a deviation, the algorithm stored in the RAM
11 is automatically corrected so that the stimulation frequency
calculated according to the algorithm at least essentially
corresponds to the measured, spontaneous heartbeat frequency.
To this end, the signal corresponding to the electrical activity of
the heart is supplied not only to the detector means 3, but also to
an arrhythmia detector 13 that supplies an output signal when an
arrhythmia is present. Such an arrhythmia detector is disclosed,
for example, in U.S. Pat. No. 3,861,387, the teachings of which are
fully incorporated herein by reference. The output signal of the
arrhythmia detector 13 is supplied via an inverter 14 to the
control logic circuit 4. The control circuit 4 thus always receives
a signal when the heart beats spontaneously following the sinue
rhythm.
A counter 15 is also present that serves for identifying the
spontaneous heartbeat frequency during phases of spontaneous heart
activity according to the sinue rhythm. To this end, a clock input
CL of the counter 15 is connected to the output of the detector
means 3. When a phase of spontaneous heart activity according to
the sinue rhythm is detected, the control logic circuit 4 supplies
a signal to a gate input G of the counter 15 for a defined
chronological duration that the control logic 4 calculates by
counting off a corresponding plurality of clock pulses of the clock
generator, said signal effecting that the spontaneous heartbeats
appearing during this chronological duration are counted.
Corresponding data are then available at the end of the time
interval at output A of the counter 15, these data indicating the
spontaneous heartbeat frequency in heartbeats per minute, since the
gate of the counter 15 is opened for one minute in the case of the
exemplary embodiment.
The output data of the counter 15 are supplied via a data line 16
to one input of a digital comparator 17. The other input of the
comparator 17 is supplied, via the data bus 12, with those data
that correspond to that stimulation frequency that is stored in the
RAM 11 for the digital output data of the analog-to-digital
converter 10 that corresponds to the momentary physical activity of
the living being and, consequently, would be set if a stimulation
of the heart 2 were required. When the comparison at the end of the
described counting event yields a deviation of the measured,
spontaneous heartbeat frequency from the stimulation frequency
stored in the RAM 11 for the momentary physical activity of the
patient, the control logic 4 recognizes this with reference to the
corresponding output signal of the comparator 17. In reference
thereto, the control logic 4 switches the RAM 11 to write mode and
connects the data line 16 to the data bus 12 via a tristate
input/output buffer 18 having a suitable number of channels. A
control input CE of the buffer 18 is connected to the W/R input of
the RAM 11. In view of the foregoing, the data corresponding to the
spontaneous heartbeat frequency measured with the counter 15 are
written into the RAM 11 instead of the previously existing data.
After the end of the write event, the control logic 4 switches RAM
11 back to read mode, disconnects the data line 16 from the data
bus 12 with the circuit 18 and resets the counter 15 by supplying a
pulse to reset input R of the counter 15.
Insofar as the output signal of the inverter 14 continues to
indicate the presence of a phase of spontaneous heart activity
according to the sinue rhythm, the control logic 4 restarts the
procedures set forth above by supplying a corresponding signal to
the gate input G of the counter 15.
It thus becomes clear that the data stored in the RAM 11 represents
an algorithm for calculating a stimulation frequency adapted to the
physical activity of the living being that is continuously adapted
to the respective conditions. The stimulation frequency calculated
according to the algorithm stored in the RAM 11 thus corresponds to
the greatest possible degree to that heartbeat frequency with which
the heart 2 would spontaneously beat.
When the comparison shows that the measured, spontaneous heartbeat
frequency coincides to the stimulation frequency stored in the RAM
11 for the digital output data of the analog-to-digital converter
10 corresponding to the momentary physical activity of the patient,
the control logic 4 resets the counter 15 by supplying a
corresponding pulse to its reset input R. Further, the control
logic 4 starts a new counting event by supplying a corresponding
signal to the gate input G of the counter 15. A switching of the
RAM 11 to write mode is omitted.
When a phase of spontaneous heart activity according to the sinue
rhythm ends during one of the above-described counting events that
serves the purpose of identifying the spontaneous heartbeat
frequency, the counting event is aborted because the control logic
4 inhibits the gate input G of the counter 15 and resets the
counter 15 by supplying a corresponding pulse to the reset input R
thereof.
In the case of the described heart pacemaker, a further digital
comparator 19 is provided having one input connected to the data
line 16 so that data corresponding to the respectively measured,
spontaneous heartbeat frequency are supplied to it. Data that
correspond to the upper end or to the lower limit value between
which the stimulation intensity can be set are supplied to the
other input of the comparator 19 by the control logic 4 via a data
line 20. Before the control logic 4 initiates the storing of a
measured, spontaneous heartbeat in the RAM 11 in the way set forth
above, a check is carried out via two successive comparison
procedures with the comparator 19 to see whether the measured,
spontaneous heartbeat frequency lies within the limits set by the
upper and lower limit values of the stimulation frequency. When the
measured spontaneous heartbeat frequency falls outside of the
predefined range, the storing thereof is omitted, so that it is
assured that no data that are physiologically meaningless or,
respectively, inadmissible can be supplied to the RAM 11. Instead,
a new counting event is started because the control logic first
rests the counter 15 by supplying a corresponding pulse to its
reset input R and then supplies the gate input G of the counter
with a signal that initiates a new counting event.
As further illustrated, a telemetry circuit 21 is connected to the
control logic 4 so that the heart pacemaker is able to
bidirectionally exchange data with an external device (not shown),
such as a programmer. This is indicated by the double arrow 22.
There is thus the possibility of programming the heart pacemaker.
For example, the upper and the lower limit value for the
stimulation frequency can be input by the programmer via the
telemetry circuit 21.
Insofar as, differing from the described heart pacemaker, the data
supplied by the counter 15, the RAM 11 and the control logic 4 that
refer to stimulation pulses or heartbeats per minute, as in the
case of the exemplary embodiment, are not compatible with one
another, these data cannot be directly supplied to the comparators
17 and 19. On the contrary, the compatibility of the data must
first be produced with, for example, a calculating event carried
out by the control logic 4.
Although the invention has been set forth with reference to a heart
pacemaker, it can also be employed in other devices for stimulating
a physiological function. In this case, but in heart pacemakers as
will, the functions critical for the invention can also be realized
in a fashion deviating from the exemplary embodiment set forth
above. In particular, other sensor devices can be employed.
Moreover, the means for implementing the algorithm for calculating
the stimulation intensity adapted to the respective physical
activity of the patient need not necessarily comprise a write/read
memory 11.
* * * * *